Method and system for providing a downlink connection in a cellular network

ABSTRACT

The present invention relates to a method and system for providing a downlink connection in a cellular network. A feedback information indicating a selected cell is transmitted to a central network element ( 20 ) controlling at least two network elements (N 1 -N 3 ) serving cells (C 1 -C 3 ) of the cellular network. The at least two network elements are controlled by the central network element based on the feedback information so as to establish the downlink connection. Thus, the downlink transmissions of the non-central network elements (N 1 -N 3 ) are controlled by the network so as to decrease performance loss due to reception errors of the feedback information. The feedback information may be a temporary ID obtained in a site selection diversity transmission control scheme.

FIELD OF THE INVENTION

The present invention relates to a method and system for providing adownlink connection in a cellular network, such as a radio accessnetwork of a WCDMA (Wideband Code Division Multiple Access) system.

BACKGROUND OF THE INVENTION

In 3^(rd) generation WCDMA systems, the downlink capacity is a limitingfactor for system capacity. Therefore, site selection diversity transmitpower control (SSDT) has been proposed as a macro-diversity method forthe soft handover mode in radio access networks.

During soft handover, a terminal device, such as a mobile station oruser equipment, is in the overlapping cell coverage area of e.g. twosectors belonging two different base stations, which are called Node Bsin the corresponding 3^(rd) generation partnership project (3GPP)specifications. The terminal device monitors received signalsbroadcasted from the different base stations, compares them to a set ofthresholds, and reports them accordingly back to the base stations.Based on this information, the network orders the terminal device to addor remove base station links from its active set of soft handover cells.The active set is defined as a set of base stations or active cells fromwhich the same user information is sent to the user equipment (UE).Furthermore, in a micro diversity or softer handover case, soft handoveris performed between sectors or cells belonging to the same base stationor node B. Thus, in the present example, the communications between themobile station and the base station may take place concurrently via twoair interface channels, one for each sector or active cell separately.This requires the use of two separate codes in the downlink direction,so that the mobile station can distinguish the signals. The SSDToperation can be summarized as follows.

The mobile station selects at least one of the cells from its active setto be “primary”, all other cells are classed as “secondary”. The mainobjective is to transmit on the downlink from the primary cell, thusreducing the interference caused by multiple transmissions in the softhandover mode. A second objective is to achieve fast site selectionwithout network intervention on higher protocol layers, thus maintainingthe advantage of the soft handover.

In order to select at least one primary cell, each cell is assigned atemporary identification (ID) and the mobile station periodicallyinforms a primary cell. ID to the active cells. In response thereto, thenon-primary cells selected by the mobile station switch off theirtransmission power. The primary cell ID is delivered by the mobilestation to the active cells via an uplink FBI (Feedback Information)field. Thus, each cell is given a temporary ID during SSDT and this IDis utilized as a site selection signal. The ID is given a binary bitsequence and the ID codes are transmitted aligned to the radio framestructure.

The mobile station selects a primary cell periodically by measuring theReceived Signal Code Power (RSCP) of common pilot channels (CPICHs)transmitted by the active cells. The cell with the highest CPICH RSCP isselected as a primary cell. Also the Signal-to-Interference Ratio (SIR)could be used for primary cell selection.

The mobile station periodically sends the ID code of the primary cellvia predetermined portions of the uplink FBI field assigned for SSDT use(FBI S field). A cell recognizes its state as non-primary if thefollowing conditions are fulfilled simultaneously:

-   1. the received ID code does not match with the own ID code;-   2. the received uplink signal quality satisfies a quality threshold    Q_(th) defined by the network; and-   3. if uplink compressed mode is used, less than N_(ID)/3 bits are    lost from the ID code (as a result of uplink compressed mode), where    N_(ID) denotes the number of bits in the ID code.

Otherwise, the cell recognizes its state as primary.

The state of the cells (primary or non-primary) in the active set isupdated synchronously.

Thus, in SSDT, a mobile station periodically chooses at least one of itsactive cells or base stations having minimum path loss in itstransmission to the mobile station. However, since the ID is sent overthe air interface, it may be possible that the ID is detectederroneously. When the ID is detected erroneously, the problem may occurthat all active base stations switch off their output powersimultaneously. On the other hand, the mobile station may receive adownlink transmission signal from an assumed primary base station, whichhowever has not transmitted the data, while it was transmitted by asecondary base station. The first problem may cause frame errors butdoes not lead to additional interference, whereas the latter problem isa more serious problem, because in this situation, the fast transmitpower control takes up the control of the transmission power of theundesired secondary base station. The resulting high transmission powerof the undesired secondary base station might cause high additionalinterference to other users.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand system for establishing a downlink connection, by means of which thedownlink capacity can be improved without increasing the risk ofinterference.

This object is achieved by a method of providing a downlink softhandover connection in a cellular network, wherein a cell is selected asa primary cell determined in a site selection power control scheme, saidmethod comprising the steps of:

-   providing a selection function for selecting said cell of said    cellular network based on a comparison of properties of downlink    signals received from at least two cells;-   transmitting a feedback information indicating the result of said    selection to a network element controlling said at least two cells;    and-   controlling said at least two cells based on said feedback    information so as to establish said downlink connection.

Additionally, the above object is achieved by a system for providing adownlink connection in a cellular network, said system comprising:

-   a terminal device having a selection function for selecting a cell    of said cellular network based on a comparison of downlink signals    received from at least two first network elements serving respective    cells, wherein said terminal device is adapted to transmit a    feedback information indicating the result of said selection to said    cellular network; and-   a second network element for receiving said feedback information and    for controlling said at least two first network elements based on    said feedback information so as to establish said downlink    connection.

Furthermore, the above object is achieved by a network element forproviding a downlink connection in a cellular network, said networkelement comprising:

-   receiving means for receiving a feedback information from a terminal    device; and-   extracting means for extracting said feedback information and for    transmitting said extracted feedback information to said cellular    network.

Finally, the above object is achieved by a network element for providinga downlink connection in a cellular network, said network elementcomprising:

-   receiving means for receiving a feedback information from a terminal    device via at least two first network elements;-   checking means for checking an identification information provided    in said feedback information; and-   controlling means for controlling said at least two first network    elements so as to establish said downlink connection.

Accordingly, the detection accuracy can be improved significantly due tothe fact that the network centrally controls the downlink transmissionsof the base stations. This improved detection accuracy due to thecentral control leads to a better overall system capacity. Inparticular, the feedback information of more than one cell or basestation of the active set of a concerned terminal device is combined toensure correct detection of the temporary ID. Thereby, performance lossdue to ID reception errors can be decreased. The proposed solution canbe regarded as an additional diversity gain to ID detection. Moreover, ashorter ID code can be used due to the improved detection probability,to thereby reduce the communication delay between the first and secondnetwork elements.

According to an advantageous further development, the identificationinformation may be extracted at the second network element, while thefeedback information is routed via the first network elements.

According to another advantageous development, the identificationinformation may be extracted at the first network element andtransmitted to the second network element.

The selected cell may be a primary cell determined in the site selectiondiversity control scheme.

Furthermore, the controlling step may comprise transmitting atransmission command to one of the at least two first network elements,which one serves the selected cell.

The downlink connection may be provided via a Dedicated Physical DataChannel of a WCDMA system.

Preferably, the controlling step may comprise a decision step based on alikeness function, the decision step being used to determine anestimated primary cell for the downlink connection. In this case, thelikeness function may be included in the feedback information. Inparticular, the decision step may be based on a predetermined decisionrule in which individual likeness functions of the active cells aresummarized to determine the index of the estimated primary cell.

The first network element may be a Node B or a base station transceiver,and the second network element may be a radio network controller.Alternatively, in a micro diversity case (i.e. softer handover), thefirst network element may be a sector of a Node B, and the secondnetwork element may be a radio network controller.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the present invention is described in more detailbased on preferred embodiments with reference to the accompanyingdrawings in which:

FIG. 1 shows a schematic block diagram of a network architecture inwhich the present invention can be applied;

FIG. 2 shows block diagrams of network elements involved in the systemaccording to the preferred embodiments;

FIG. 3 shows a signaling diagram indicating an SSDT signaling accordingto a first preferred embodiment; and

FIG. 4 shows a signaling diagram indicating an SSDT signaling accordingto a second preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments will now be described on the basis of a radioaccess network architecture of a 3^(rd) generation WCDMA system, such asa Universal Mobile Telecommunications System Terrestrial Radio AccessNetwork (UTRAN), as indicated in FIG. 1.

According to FIG. 1, a user equipment 10 is connected via radiointerfaces to a radio network sub-system (RNS) of the UTRAN. The RNScomprises e.g. three Node Bs N1, N2, N3 which are arranged to route thedata flow to a Radio Network Controller (RNC) 20. It is noted that theterm “Node B” may be replaced by the more generic term “Base Station”which has the same meaning. The Node Bs N1 to N3 are adapted to serverespective cells C1 to C3, while the ID of at least one of them may bestored in the active set of the UE 10 due to their overlapping cells.The RNC 20 owns and controls the radio resources in its domain, i.e. theNode Bs N1 to N3 connected to it. It provides a radio resource controland mobility management function and is the service access point for allservices the UTRAN provides to at least one core networks (indicated onthe upper part of FIG. 1).

According to the preferred embodiments, the performance loss due to theID reception errors is decreased by shifting the SSDT control towardsthe network site, e.g. to the RNC 20, such that the radio access networkcontrols the Dedicated Physical Data Channel (DPDCH) or DedicatedPhysical Channel (DPCH) transmission of the Node Bs N1 to N3.

FIG. 2 shows a schematic block diagram of the network elements involvedin the SSDT transmission between the UE 10 and the RNC 20. A UE 10transmits its feedback information indicating temporary IDs of primarycells in the FBI field of the DPDCH channel. The Node Bs N1 to N3 thentransmit the received ID softbits or softvalues (FBI bits) to the RNC20. In FIG. 2, relevant parts of the preferred embodiments are shown ina first Node B N1 and the RNC 20. In particular, the first Node B N1comprises a transceiver (TRX) 12 for transmitting and receiving datato/from a UE 10. The received data is supplied to a FBI extraction unit14 where the information contained in the FBI field is extracted andsupplied or transmitted to the RNC 20. It is noted that the same partsor blocks are also provided at the other node Bs N2 and N3. The RNC 20comprises a switching function or switch 22 for selecting at least oneof the Node Bs N1 to N3 and for supplying data received from theselected Node B to an ID checking function or unit 24, where a primarycell to be used for the downlink transmission to the UE 10 is estimatedor determined, e.g. based on a decision rule. The identity or index orID of the estimated primary cell is supplied to an SSDT commandgeneration unit 26 in which corresponding commands for switching on oroff at least one of the respective Node Bs N1 to N3 are generated andsupplied to the switch 22. Thus, the SSDT control is performed at theRNC 20 to thereby reduce the probability of the ID reception errors.

It is noted that the above described functions or blocks of the node BsN1 to N3 and the RNC 20 may be implemented by discrete hardware elementsor by software routines controlling a processor device.

FIG. 3 shows a signaling diagram of an SSDT signaling according to thefirst preferred embodiment. In the first preferred embodiment, acentralized detection of the ID codes of determined primary cells isperformed. The Node Bs N1 to N3 transmit the received ID softbits orsoftvalues extracted at the FBI extraction unit 14 to the RNC 20. TheRNC 20 detects the ID aid MRC (Maximal Ratio Combining) between IDsoftbits or softvalues originated from one active set. It is noted thatthe term “active set” means the set of Node Bs which transmit for one UEor mobile terminal during a normal soft handover operation. Thisdetection is performed at the ID checking unit 24. After ID detection,the RNC 20 informs the Node Bs N1 to N3 to switch off or on theconcerned DPDCH or DPCH transmission by using the SSDT commandgenerating unit 26.

Thus, as indicated in FIG. 3, the FBI bits are initially transmittedfrom each of the Node Bs N1 to N3 to the RNC 20. Then, after IDdetection, respective SSDT commands are transmitted to each of the NodeBs N1 to N3.

FIG. 4 shows a signaling diagram indicating an SSDT signaling accordingto the second preferred embodiment. In the second preferred embodiment,the Node Bs N1 to N3 are arranged to detect the temporary ID of theprimary cells e.g. in the FBI extracting unit 14. The detected IDs arethen transmitted to the RNC 20. Based on the received IDs, the RNC 20checks in the ID checking unit 24 whether there is existing at least onetransmitting Node B in the active set. Based on the checking result, theRNC 20 generates corresponding switching commands in the SSDT commandgenerating unit 26 and transmits these commands to one or all Node Bs inthe active set. This decentralized ID detection may also be performedbased on a decision rule e.g. as explained in the following.

The decision rule may be based on some kind of likeness functions whichmay be applied in the RNC 20 in the case of the first preferredembodiment, or in the Node Bs N1 to N3 in the case of the secondpreferred embodiment. In the second preferred embodiment, the likenessfunctions may be sent to the RNC 20, instead of the ID softbits. Then,the RNC 20 makes the decision in the ID checking unit 24 based on thereceived likeness functions.

In the following, the ID detection based on a soft variable connectionbetween a Node B and the RNC 20 is described based on correspondingalgorithms. In the conventional SSDT schemes, the temporary ID isdetected separately at each Node B. In this case, the decision ruleapplied can be expressed as follows: $\begin{matrix}{z^{i} = {\arg\quad\underset{z \in {\{{1,\ldots\quad,b}\}}}{\max\quad\Omega^{i}}(z)}} & (1)\end{matrix}$where z^(i) is the index of the estimated primary cell in the i^(th)Node B and b is the number of active cells. The likeness function of theabove example can be expressed e.g. by the following equation:$\begin{matrix}{{\Omega^{i}(z)} = {\sum\limits_{d = 1}^{{ID\_ code}{\_ length}}{{q^{i}\lbrack d\rbrack}{c_{z}\lbrack d\rbrack}}}} & (2)\end{matrix}$where q[d] is the soft decision from the received d^(th) ID code symbolsand c_(z)[d] is the d^(th) ID code symbol from the z^(th) ID codes.

In contrast thereto, according to the preferred embodiments of thepresent invention, the temporary ID is detected centrally at the RNC 20.The applied decision rule can be expressed as follows: $\begin{matrix}{z = {\arg\quad\max\underset{z \in {\{{1,\ldots\quad,b}\}}}{\left( {\sum\limits_{i = 1}^{b}{\Omega^{i}(z)}} \right)}}} & (3)\end{matrix}$where z is the index of the estimated primary cell.

As already mentioned, the likeness function can be calculated in theNode Bs or at the RNC 20 and may be obtained by the above equation (2).

The performance difference between the preferred embodiments of thepresent invention and the conventional SSDT schemes is described in thefollowing based on practical examples. In the example, a three-way softhandover is assumed, where three Node Bs are contained in the activeset, as indicated in FIG. 1. Assuming an ID code length of 16 bits, aradio channel environment of a pedestrian, a UE location at the boundaryof cells, a speech signal of 12.2 kbit/s and equal pathloss in eachlink, a gain of 1.2 to 2.3 dB compared to the conventional SSDT solutioncan be obtained. Due to the strong decrease in the ID reception errors,the performance of the SSDT scheme according to the present inventioncan be regarded as an SSDT performance with perfect ID feedback.

In conventional SSDT solutions, the most serious error situation existswhen the primary ID is detected erroneously in the primary Node B orbase station. This may occur due to fast fading situations which arefully uncorrelated between the uplink and downlink direction. In theSSDT scheme according to the present invention, the probability of thiskind of error is significantly lower due to the additional diversitygain in the centralized ID detection.

In the present invention, communication delays between the Node Bs N1 toN3 and the RNC 20 may become a problem. However, this problem can betackled for example by using a shorter ID code. Nevertheless, thisshorter ID code does not lead to a deterioration of the performance, asthe detection probability is improved.

It is noted that the present invention can be implemented in anycellular network in which some kind of macro diversity functionality isprovided. The names of the various functional entities, such as the RNC20 or the Node Bs N1 to N3 may be different in different cellularnetworks. The names used in the context of the preferred embodiment arenot intended to limit or restrict the invention. Moreover, any kind ofdecision rule may be applied to determine or estimate the cell to beused for the downlink transmission at the central network element, i.e.the RNC 20, or at the non-central network elements, i.e. the Node Bs N1to N3. The preferred embodiments may thus vary in the scope of theattached claims.

1. A method of providing a downlink soft handover connection in acellular network, wherein a cell is selected as a primary celldetermined in a site selection power control scheme, said methodcomprising the steps of: a) providing a selection function for selectingsaid cell of said cellular network based on a comparison of propertiesof downlink signals received from at least two cells (C1-C3); b)transmitting a feedback information indicating the result of saidselection to a network element (20) controlling said at least two cells(C1-C3); and c) controlling said at least two cells (C1-C3) based onsaid feedback information so as to establish said downlink connection.2. A method according to claim 1, further comprising the step of addingan identification information of said selected cell to said feedbackinformation, and extracting said identification at said network element(20).
 3. A method according to claim 1, further comprising the step ofadding an identification information of said selected cell to saidfeedback information, and extracting said identification information atsaid at least two cells (C1-C3), and transmitting said extractedidentification information to said network element (20).
 4. A methodaccording to claim 1, wherein said controlling step comprisestransmitting a transmission command to one of said at least two cells(C1-C3), which one serves said selected cell.
 5. A method according toclaim 1, wherein said downlink connection is provided via a DedicatedPhysical Data Channel (DPDCH) or a Dedicated Physical Channel (DPCH) ofa WCDMA system.
 6. A method according to claim 1, wherein saidcontrolling step comprises a decision step based on a likeness function,said decision step being used to determine an estimated primary cell forsaid downlink connection.
 7. A method according to claim 6, wherein saidlikeness function is included in said feedback information.
 8. A methodaccording to claim 7, wherein said decision step is based on thefollowing decision rule:${z = {\arg\quad\max\underset{z \in {\{{1,\ldots\quad,b}\}}}{\left( {\sum\limits_{i = 1}^{b}{\Omega^{i}(z)}} \right)}}},$wherein z denotes the index of said estimated primary cell for saiddownlink connection, b denotes the number of active cells, and Ω^(i)denotes the likeness function for the i-th one of said at least twocells.
 9. A system for providing a downlink connection in a cellularnetwork, said system comprising: a) a terminal device (10) having aselection function for selecting a cell of said cellular network basedon a comparison of downlink signals received from at least two firstnetwork elements (N1-N3) serving respective cells (C1-C3), wherein saidterminal device (10) is adapted to transmit a feedback informationindicating the result of said selection to said cellular network; and b)a second network element (20) for receiving said feedback informationand for controlling said at least two first network elements (N1-N3)based on said feedback information so as to establish said downlinkconnection.
 10. A system according to claim 9, wherein said firstnetwork element is a Node B (N1-N3) or Base Station Transceiver, andsaid second network element is a Radio Network Controller (20).
 11. Asystem according to claim 9, wherein said first network element is asector of a Node B (N1-N3), and said second network element is a RadioNetwork Controller (20).
 12. A system according to claim 9, wherein saidcellular network is a WCDMA radio access network.
 13. A network elementfor providing a downlink connection in a cellular network, said networkelement comprising: a) receiving means (12) for receiving a feedbackinformation from a terminal device (10); and b) extracting means (14)for extracting said feedback information and for transmitting saidextracted feedback information to said cellular network.
 14. A networkelement according to claim 13, wherein said network element is a Node B(N1-N3) or a Base Transceiver Station.
 15. A network element forproviding a downlink connection in a cellular network, said networkelement comprising: a) receiving means (22) for receiving a feedbackinformation from a terminal device (10) via at least two first networkelements (N1-N3); b) checking means (24) for checking an identificationinformation provided in said feedback information; and c) controllingmeans (26) for controlling said at least two first network elements(N1-N3) so as to establish said downlink connection.
 16. A networkelement according to claim 15, wherein said network element is a radionetwork controller (20).